The present invention relates generally to systems and methods for image acquisition, and in particular to systems, methods, and devices for reducing noise in pixels used in image acquisition.
FIG. 1 illustrates a conventional metal oxide semiconductor (“MOS”) pixel 100. As illustrated, pixel 100 includes a reset transistor 110 connected to photodiode 150, a source follower transistor 130, and an access transistor 140. During reset of pixel 100, a reset signal 160 associated with a pixel (i) is asserted high such that the rail voltage associated with conventional pixel 100 is applied to the gate of reset transistor 110. Prior to application of reset signal 160, reset transistor 110 is cut off.
Upon assertion of reset signal 160, reset transistor 110 provides a low impedance path between a node 191 and a node 151. This low impedance path causes a reverse bias on photodiode 150, where the voltage at node 191 is greater than a common ground 193. This results in a charge build-up, and corresponding voltage at node 151 that remains after reset signal 160 is deasserted and node 151 is isolated from node 191. When incident light 194 falls upon photodiode 150, the voltage at node 151 begins to decay at a rate defined by the photocurrent through photodiode 150, that corresponds to the amount of incident light 194 impinging upon photodiode 150. Thus, where a high incidence of light is detected, voltage at node 151 decays rapidly, and where only a low incidence of light is detected, voltage at node 151 decays slowly. The voltage at node 151 is tied to the gate of source follower transistor 130.
Source follower transistor 130 provides a signal at its source that is representative of the voltage at node 151. Further, when the incidence of light on photodiode 150 is to be read, a select signal 180 is asserted high, thus providing a low impedance path from node 141 to a bit sense node 170. Thus, where a sufficient amount of incident light 194 impinges upon photodiode 150, bit sense node 170 will exhibit a low voltage, otherwise, bit sense 170 will exhibit a voltage corresponding to the voltage at node 151.
Various problems associated with such MOS pixels are discussed in “Analysis of Temporal Noise in CMOS Photodiode Active Pixel Sensor” by Tian et al., IEEE Journal of Solid State Circuits, Vol. 36, No. 1, January 2001. More particularly, Tian et al. disclose that MOS image sensors suffer from significant noise problems due to the design of such sensors. Such noise can be associated with operation of reset transistor, while other noise derives thermal and shot noise sources in pixel 100. Some approaches to control the various noise sources include operating reset transistor 110 in a subthreshold range, however, such approaches can result in image lag.
Alternatively, attempts have been made to reduce such noise by, for example, overdriving the gate of reset transistor 110 or by using a PMOS instead of an NMOS type transistor for reset transistor 110. However, such approaches can result is a dramatic increase in reset noise power. Such noise hampers the proper operation of such image sensors. Further, such noise can become more significant as the size of pixel 100 is reduced.
In some cases, off-chip digital signal processing has been employed to allow a sufficiently small pixel 100, while maintaining noise levels at acceptable level. In other cases, pixel 100 is simply comprised of relatively large devices that are less susceptible to noise constraints. Such an approach, however, is contrary to the industry need to constantly reduce pixel size. This industry need is highlighted in “CMOS Image Sensors: Electronic Camera-On-A-Chip” by Fossum, IEEE Transactions on Electronic Devices, Vol. 44, No. 10, October 1997. More particularly, Fossum points out the rapid trend toward reduced size pixels 100. This size reduction is particularly important where low powered devices are desired, such as in cellular telephone and other mobile applications. Thus, there exists a need in the art to provide a pixel that exhibits reduced noise characteristics and/or reduced size characteristics.